Parallel Universes

I just recently read an article from Scientific American (May 2003 issue and on their website here). I'm curious to know if the theories in the article are, in fact, how many theoretical physicists feel about our universe.

Essentially, the article claims that "the simplest and most popular cosmological model today predicts that you have a twin in a galaxy about 10 to the 10^28 meters from here." The article then goes on to claim that our "Hubble volume" (everything we can see) is actually a universe virtually swimming in an unending sea of other universes and there are several levels of universes layed upon this one (the sea of universes or as the writer calls it, the "Level I Multiverse").

Well, I can't really sum it up much more than that. (You'd have to read the article to get the full gist of it.) But my main concern was that they state several times that this is the "most popular cosmological model", which shocked me because I had never read/heard of it before. The fact that there is only a (rather LARGE) physical distance between "our universe" and another universe made me think that this would NOT be a fairly popular model.

Yeah, that's basically right, though I wouldn't use the terms 'parallel universe' or 'multiverse.' The standard cosmology is that the universe is infinite in size, though there are a couple of less popular alternatives.

When you have infinite size, it's like the "100 monkeys banging on a hundred typewriters" producing the works of Shakespeares. If there are infinite number of galaxies, then eventually you will find one that looks as close as you want to our own, under a few reasonable assumptions.

The idea of different sections of the universe having different laws of physics is also common, but not as widely accepted.

Multiple universes, parallel universes? I don't use those terms often, but I do use "multiple histories" because it makes more sense to me.
Anyway, the idea of multiple histories is just that there are several probable ways that something could turn out. You could have decided to sit on a bench, that you didn't know was wet with paint, or decided not to, or looked at the bench and realized it was wet with paint, etc. Possible histories "branch out" of this simple occurance, depicting all the possibilities. And in many cases, those possibilities are infinite.
Damgo, I believe you shouldn't take the idea of "parallel universe" litrally. Maybe parralel time coordinates or something like that is a better name.

Originally posted by MajinVegeta Multiple universes, parallel universes? I don't use those terms often, but I do use "multiple histories" because it makes more sense to me.
Anyway, the idea of multiple histories is just that there are several probable ways that something could turn out. You could have decided to sit on a bench, that you didn't know was wet with paint, or decided not to, or looked at the bench and realized it was wet with paint, etc. Possible histories "branch out" of this simple occurance, depicting all the possibilities. And in many cases, those possibilities are infinite.
Damgo, I believe you shouldn't take the idea of "parallel universe" litrally. Maybe parralel time coordinates or something like that is a better name.

Originally posted by damgo The standard cosmology is that the universe is infinite in size, though there are a couple of less popular alternatives.

I don't understand how the universe could be infinite in size and still agree with the idea of a big bang. If the universe were once to exist as a singularity with such a tiny volume, how then did it become infinitely huge? This would require an infinitely huge rate of expansion, thus infinitely separating distances in space (the sun and the earth, for example).

I happened to read that article as well, and although it was interesting, I thought their concepts of parallel uinverses were a bit skewed. They gave an example of another "you" living 10^(28) meters away, but this is not a parallel universe, it is merely a section of the universe that resembles our own section of the universe. Both "universes" are on the same spatial plane. I tend to like the quantum many-worlds theory, measuring distances between universes in terms of quantum decisions rather than space...

The standard explanation of the Big Bang as starting with a "tiny compressed ball of matter" is, um, just plain wrong. If the universe is infinite, it was always infinite, up to the singularity itself, which you can't say anything about.

Think of space itself as expanding, not stuff being exploded. eg, imagine a 1-D line, like a number line. Objects are positioned at say each integer location k={..., -2, -1, 0, 1, 2,...} right now, which we'll say is t=1. Let the position of those objects change in time as x_k(t)= k*t .

So density increases as 1/time. But as you go back towards the Big Bang -- t=0 -- things get ever closer together and more and more dense, though the universe is still always infinite.

This is not at all an accurate picture -- it's far more complicated -- but it's the basic idea of how you can have infinite size coming from a singularity with infinite density.

If there are infinite number of galaxies, then eventually you will find one that looks as close as you want to our own

I don't understand how this can be possible. Imo this is task for mathmatics to prove or disprove. As I see it, any particle in universe is like bit of information. Number of such bits can form multitudes of combinations. Like if you have 4 bits, you can have 2^4 combinations. If you have 10^81 bits, you have 2^10^81 combinations before repeating is unavoidable.

Basically, expanding universe means expanding possible combinations, and amount of combinations rises exponentially faster than number of bits. I can't possibly see, how would twin universe be possible in the face of that... Probability of having exact twin drops inversely proportionally to number of possible combinations, doesn't it?